How to Ensure the Welding Quality of Zirconium Alloy

In the previous article, we introduced the basic properties of zirconium alloys and the problems that easily occur during the welding process. Next, we will explain how to ensure the welding quality of zirconium alloys and some precautions.

welded zirconium

Precautions for zirconium alloy welding

  • In the welding prefabrication stage of a large number of welds, a special closed clean place needs to be set up, and strict control of environmental dust pollution and air humidity. For example, when entering the construction site, measures such as wearing clean labor insurance shoes must be worn to ensure the cleanness of the welding environment. In the outdoor installation environment, make a temporary operating room to achieve clean conditions.
  • Strengthening the requirements for the weld joint groove and within 70mm of both sides of the groove and the cleanliness of the surface of the welding wire is an important factor to ensure the welding quality.
  • In the welding process of zirconium alloy, pores are the most prone to defects, and it is mostly concentrated near the fusion line and the centerline of the weld. The most critical steps to prevent the occurrence of welding porosity defects are to strengthen the control of the cleanliness and humidity of the welding environment, and to enhance the cleaning of the bevel and the surface of the welding material, so as to improve the quality of the internal and external protection of high purity argon in the weld zone.
  • The zirconium alloy has a low thermal expansion coefficient, a small amount of thermal deformation, and a small volume change during phase change. It has a low content of impurities such as sulfur, phosphorus, and carbon, so there is no obvious tendency to form cracks during welding. However, when the welding seam absorbs a certain amount of oxygen, nitrogen and hydrogen gas impurities, the performance of the welding seam and the heat-affected zone will become brittle. If there is stress in the weld in the peer group, a cold crack will occur. In addition, the hydrogen atoms have the property of diffusing and accumulating to the high-stress parts in the heat-affected zone at a relatively low temperature, which promotes the formation of relatively weak links in these parts, which may lead to the occurrence of delayed welding cracks.
  • In the welding test, manual tungsten argon arc welding with low welding line energy and convenient gas welding protection should be selected; The larger-diameter welding torch nozzle, the outer surface of the weld seam, and the internal argon filling method of the pipe are used for air isolation to achieve the purpose of the weld seam not being oxidized and absorbing harmful gases.
  • The filler wire used for zirconium alloy welding should be selected according to the principle of matching the composition of the base metal. The surface of the welding wire must be free from defects such as heavy skin, cracks, oxidation, and metal or non-metallic inclusions. The welding wire should be cleaned and dried before use.
  • Zirconium alloy tungsten arc welding requires high-purity argon with a purity of not less than 99.999%, and its impurity content meets the requirements of the current GB / T4842 standard. Due to the extremely high requirements for the purity of the welding protective gas, the welding process needs to be continuously inflated and cannot be interrupted halfway, otherwise, the argon filling must be replaced again. The method of using an ordinary single bottle of argon direct gas supply cannot meet the protection requirements. Multiple bottles of argon gas need to be connected in series to increase the gas supply capacity, and multiple welders can be operated simultaneously by dividing the cylinder.
  • Because zirconium alloys are active at high temperatures, relying solely on the argon gas supplied by the argon arc welding torch nozzle to protect the molten pool and high-temperature bead and heat-affected zone during welding cannot guarantee the welding quality. In order to ensure that the requirements for gas isolation in high-temperature areas and prolonged argon protection time are met, special external gas protection devices for pipes must be added to provide high-purity argon isolation protection for weld pools, high-temperature weld beads and heat-affected zones at high temperatures.

Stanford Advanced Materials supplies high-quality zirconium alloy products to meet our customers’ R&D and production needs. Please visit https://www.samaterials.com/ for more information.

How Did Nuclear Zirconium Alloys Develop?

Zirconium alloys have a small thermal neutron capture cross-section (0.185b) and are surprisingly resistant to corrosion, so they are widely used in fission reactors, such as core-clad tubes, grids, and guide tubes in boiling water reactors, as well as pressure pipes and exhaust reactor vessels in pressurized water reactors.

Nuclear zirconium alloy

With the application of zirconium alloys in the nuclear energy industry, the zirconium industry has developed rapidly.

In the nuclear giant change reactor, nuclear fuel is fission reaction all the time. In the reaction, the neutron bombards the nucleus of U235, which splits into Ba140 and Kr93, and releases two or three neutrons at the same time; other U235 nuclei are bombarded by these neutrons and re-fission. This is the chain reaction of fission.

nuclear-reactor
Nuclear Reactor

A material with a large neutron capture cross section will absorb many neutrons when they hit the wall, reducing the efficiency of the chain reaction. Meanwhile, the chain reaction produces a lot of heat, which is removed by circulating cooling water (or other coolants) to avoid overheating and damage to the reactor. When metals come into contact with high-temperature water, they can be corroded (oxidized). Materials with poor corrosion resistance need to be replaced frequently, which increases the cost and easily leads to safety accidents. Therefore, as core-cladding and structural materials, zirconium alloys are required to have low neutron capture cross-section and excellent corrosion resistance, so the development of zirconium alloys should be attributed to the nuclear industry.

Origin of zirconium alloys

Initially, zirconium was not considered a suitable material for use in the nuclear industry, because studies have shown that zirconium’s effect on thermal neutron absorption can affect the efficiency of nuclear reactors. Later, researchers at the Oak Ridge Institute found that 2.5% of the hafnium in zirconium was responsible for its large thermal neutron capture cross-section.

Zirconium Alloy
Zirconium Alloy

Zirconium and hafnium are associated with ore and are generally difficult to separate. Until the 1850s, Admiral in the Naval Nuclear Propulsion project decided to use zirconium in the water-cooled reactor of the Nautilus Nuclear Submarine. Although zirconium had already been used for the project by that time, there were no strict standards for the use of zirconium, and the researchers only knew that improving the purity of zirconium would be good for the properties of the alloy. Some processes are used to purify strip zirconium, but it still contains small amounts of nitrogen, making it less resistant to corrosion at high temperatures. Finally, the researchers realized that purity was not the key to zirconium’s corrosion resistance, because they found that some zirconium materials containing impurities (such as tin, iron, chromium, and nickel) were more resistant to corrosion than higher-purity zirconium materials. Therefore, the development of zirconium alloys is put on the agenda.

Development of zirconium alloys

The first alloy, Zircaloy-1, contains 2.5% tin. It was found that the corrosion rate of Zircaloy-1 alloy was increasing and not consistent with the expected decrease. This was similar to a normal sponge zirconium material, so Zircaloy-1 was quickly abandoned.

At the same time, the researchers found that adding iron and nickel to the Zircaloy-2 could improve corrosion resistance. The tin content was reduced to 1.5% and 0.15% iron, 0.05% nickel and 0.10% chromium were added. It was found that Zircaloy-2 had the same mechanical properties as Zircaloy-1, but the high-temperature corrosion resistance of Zircaloy-2 was much better than that of Zircaloy-1. However, during the service of the pressurized water reactor, the alloy produces a lot of hydrides, resulting in hydrogen embrittlement.

By studying the binding technique, the researchers found that nickel greatly enhanced the hydrogen absorption capacity of zirconium alloys. The researchers removed the nickel from the Zircaloy-2, creating a Zircaloy-3. But Zircaloy 3 was quickly abandoned because its strength was too low. In addition, Zircaloy-3 produced many striated Fe-Cr binary intermetallic compounds when it was processed in the two-phase zone, so it could not provide sufficient corrosion resistance. The strength of Zircaloy-3 was still too low, although changes in the heat treatment process prevented the production of the striated compound.

Zircaloy-2 Composition
Zircaloy-2 Composition

The researchers compensated for the nickel by increasing the iron content by 0.22 percent and found that the corrosion resistance of the new alloy was similar to that of zircaloy-2, which had only half the hydrogen absorption rate. The new alloy quickly became a major part of the pressurized water reactor, the first Zircaloy-4.

Zirconium alloys for the nuclear industry have been developed into the third generation of products, which are used in various reactors.

The first generation is the standard zircaloy-4 and Zircaloy-2, whose composition and process requirements are specified in the ASTM standard. This generation zirconium alloy is still in use.

The second generation is low tin Zircaloy-4 and optimized Zircaloy-4. The tin content of low tin Zircaloy 4 decreased from 1.2% ~ 1.70% to 1.20% ~ 1.50%, and the carbon and silicon were controlled at 0.008% ~ 0.020% and 0.005% ~ 0.012%, and the cumulative annealing process parameters in the alpha phase after quenching in the beta phase were strictly controlled; the optimized zircaloy-4 is based on the low tin zircaloy-4, and the content of alloy elements and process parameters are more strictly controlled, so as to improve the uniformity of materials.

The third generation of zirconium alloy has excellent properties and is widely used as a fuel rod cladding tube and fuel assembly guide tube. NDA and MDA from Japan, HANA from South Korea and composite casings from Siemens are also examples of this generation of products.

Prospect of zirconium alloys

Zirconium alloys above 620℃ (depending on composition) convert to body-centered cubic β-zirconium. After the transformation, the mechanical properties and corrosion resistance of the alloy will be greatly reduced, and it cannot continue to maintain the safe operation of the nuclear reactor. The famous event is the accident at the Fukushima nuclear power plant in Japan. Affected by the big earthquake in eastern Japan, the reaction water of Fukushima nuclear power plant leaked, and the cladding temperature increased significantly. The zirconium alloy cladding softened quickly, and brittle material formed with the leakage of air, leading to the leakage of nuclear fuel. Large amounts of nuclear-contaminated water flow into the sea have caused great damage to the ecology of the world.

As a nuclear reactor cladding material, it needs to have a small thermal neutron capture cross-section, which leads to the zirconium alloy cannot be highly alloyed, so it is bound to be difficult to break through the zirconium alloy’s high-temperature performance. At present, countries attach great importance to this problem. On the one hand, they are trying their best to make a breakthrough in the high-temperature performance of zirconium alloy; on the other hand, they are looking for alternative products of existing fuel cladding, such as silicon carbide (SiC) composite material, molybdenum alloy, cobalt alloy and so on. Molybdenum alloys and cobalt alloys were originally intended as structural materials for fusion reactors. Although they do not have the same low thermal neutron absorption cross-section as zirconium alloys, they have excellent high-temperature stability.

Stanford Advanced Materials supplies high-quality zirconium alloys to meet our customers’ R&D and production needs. Please visit http://www.samaterials.com for more information.

What Do You Know About Zirconium And Zirconium Alloys?

Compared with traditional iron, copper, nickel and other metal elements, zirconium has lower density and smaller thermal expansion coefficient. In addition, zirconium has a low thermal neutron absorption cross-section (only 0.18×10-28 m2) and good corrosion resistance, which makes zirconium and zirconium alloys have a wide range of applications in the nuclear industry, aerospace and other special fields.

Zirconium and its alloys have been widely used as cladding materials in nuclear reactors. Zirconium and its alloys reflect neutrons back into the reactor more efficiently than stainless steel, greatly saving uranium fuel; Zirconium alloy has good corrosion resistance in high temperature and high-pressure steam of 300 ~ 400 ℃, which also makes the reactor have a long service life. Therefore, zirconium is regarded as the first metal in the atomic age.

zr-alloy

Development status of zirconium and its alloys

Zirconium, which is found in the earth’s crust at about 220 g /t, ranks 20th, ahead of other common metals such as copper, nickel, lead, and cobalt. Initially, zirconium alloys were mainly used as cladding materials in the nuclear industry. In recent decades, zirconium alloys have been widely used in the chemical industry, medical industry, and some special fields.

Zirconium alloy for nuclear use

Zirconium alloys have been widely used in the nuclear industry because of their very low thermal neutron absorption cross-section and good resistance to high temperature and pressure corrosion. France, the United States, Germany, and Russia have developed a series of zirconium alloys for nuclear use. At present, Zr-2, Zr-4, Zr2.5nb and ZIRLO, E635, M5, and NDA zirconium alloys have been successfully applied in the nuclear industry. These newly developed zirconium alloys have lower radiation creep properties and better resistance to iodine stress corrosion. In addition, they are able to meet the requirements of high burnup of the fuel assemblies, increasing the service life of the assemblies to 30 years.

nuclear-reactor

Corrosion resistant zirconium alloy

Zirconium has excellent corrosion resistance against most organic acids, inorganic acids, strong alkalis, and some molten salts. Therefore, zirconium can be used to improve the service life of some key components in corrosive environments. Another way to improve the corrosion resistance of alloy parts is surface pretreatment. In industry, zirconium is placed in high-temperature air to obtain a dense oxide film, so as to improve the corrosion resistance and erosion resistance of zirconium and its alloys. The results show that the corrosion rate of zirconium treated by surface oxidation in sulfuric acid medium is only 5% of that of pure zirconium, but the erosion resistance is increased by twice.

At present, zirconium is widely used as corrosion resistant material in the chemical industry, and it has been widely used in the heat exchanger, dike washing tower, reactor, pump, valve, and corrosion medium pipeline. For example, zirconium alloys have been used to produce concentrated and hydrolyzed tubes in hydrogen peroxide production lines, while zirconium pressure reducing valves, agitators and flow meters are used in fertilizer production, sewage treatment, and dye industries.

Biomedical materials are a new high-tech material in recent years, and biomedical alloys must have good compatibility and corrosion resistance with the environment of biological fluids. Zirconium is valued by researchers for its good biocompatibility, elastic modulus similar to bone and corrosion resistance. Ti6Al4V, a titanium alloy implanted earlier in hard tissues of the human body, has an elastic modulus of nearly 110 GPa, which is much higher than the elastic modulus of 15 ~ 30 GPa of natural bones of the human body.

High strength zirconium alloy

In the fields of space exploration, deep-sea exploration, and high-speed railway, there are often some special operating environments, such as the alternating temperature environment of -200 ~ 200 ℃, continuous space irradiation and relative motion of structural parts, etc. Under these special circumstances, long-serving structural components are often faced with fatigue damage, dimensional instability, atomic oxygen erosion, and friction wear. At present, the structural parts used in these special fields are mainly made of 20Cr, GCr15 and other alloy steel materials, which often have problems such as poor radiation resistance, easy damage of moving parts, high density and high cost.

Compared with traditional alloy steels, zirconium and its alloys have several important potentials:

  • The thermal expansion coefficient of zirconium is small and the size structure is stable, so it has the potential to produce precise structural components;
  • It has the potential of resisting space radiation damage;
  • It has the potential to resist atomic oxygen erosion;

Therefore, zirconium and its alloys are expected to adapt to unconventional environmental conditions in special fields and have the potential to be used as structural components in special environments.

Please visit http://www.samaterials.com for more information.

Argon Arc Welding Technology of Zirconium and Zirconium Alloy

Zirconium and zirconium alloys have excellent corrosion resistance to acid and alkali, and even surpass niobium, titanium and other metals in some media. Therefore, zirconium and zirconium alloys are gradually used as structural materials such as equipment and pipelines in the chemical industry with strong corrosion resistance due to their good corrosion resistance in recent years.

Due to the high-temperature chemical activity, zirconium and zirconium alloys can react with various elements in the air at high temperature, thus damaging their mechanical properties. Therefore, in the process of zirconium and zirconium alloy welding, the key to ensuring the quality of welding is to select a clean operating environment and strengthen the isolation and protection of welding seams and parts in the heat-affected zone.

Basic properties of zirconium and zirconium alloys

Zirconium and zirconium alloy materials mainly include R60702, R60704, and R60705. Zirconium and zirconium alloys have good welding properties and stable chemical properties at room temperature. However, its high-temperature chemical properties are very active, and it has a strong affinity for the pollution of oxygen, nitrogen, hydrogen and dust and humidity in the operating environment.

The excellent corrosion resistance of zirconium and zirconium alloys comes from the oxide film formed on the surface and depends on the integrity and firmness of the oxide film. When zirconium and zirconium alloy absorb a certain amount of oxygen, nitrogen, hydrogen, and other gas impurities, their mechanical properties and corrosion resistance will decrease sharply. Therefore, strengthening the protection of the surface of environmental dust, humidity and heat affected area and the back of the welding seam is the key element of quality control in the welding process.

Zirconium and Zirconium Alloy
Zirconium and Zirconium Alloy

Factors influencing the welding quality of zirconium and zirconium alloy

  • The tendency of weld cracks

Due to the low thermal expansion coefficient of zirconium and zirconium alloy, the volume change caused by thermal deformation and phase change is very small, and the content of sulfur, phosphorus, carbon and other impurities is very low, there is no obvious trend of cracks in the welding process. However, when the welding seam absorbs a certain amount of oxygen, nitrogen and hydrogen gas impurities, the performance of the welding seam and the heat-affected zone will become brittle. If there is stress in the welding seam, cold cracks will occur.

At the same time, hydrogen atoms have the property of diffusing and aggregating to the high-stress parts in the heat-affected zone with lower temperature, which leads to the formation of relatively weak links in these parts, which may lead to the generation of welding delay cracks.

  • Selection of welding materials

The filler wire for zirconium and zirconium alloy welding should be selected according to the principle of matching the base material composition. The surface of welding wire shall not have heavy skin, crack, the oxidation phenomenon and metal or non-metal inclusion defects. Besides, the welding wire should be cleaned and dried before use.

  • Selection of protective gas

Argon arc welding with tungsten electrode of zirconium and zirconium alloy shall adopt high purity argon with 99.999% purity and the impurity content shall meet the requirements of GB/T4842 current standards.

Argon Arc Welding
Argon Arc Welding

Due to the extremely high requirements on the purity of welding protective gas, continuous gas charging is required during the welding process, and the gas cannot be interrupted in the process; otherwise, argon charging needs to be replaced again. Therefore, the direct gas supply method using ordinary argon in a single bottle cannot meet the protection requirements. It is necessary to increase the gas supply capacity of multiple argon bottles in series and satisfy the simultaneous operation of multiple welders through the air separation cylinder.

Please visit http://www.samaterials.com for more information.